Than and then are pretty common in a lot of places. But I'll grant that those might be a little harder to get right than to, too and two; your and you're; and their, there, and they're, as mentioned.
I'm not concerned so much or even bothered by another's grammatical mistakes. The point is the discussion and the substance of the points made for me.
Yeah. I usually noticed spelling and grammar mistakes, but I never comment on them. First off, this is the internet - I'll let the teachers at school/college correct someone's spelling and grammar mistakes (though not everyone is in school). Second, yes, as long as the overall point and message is clear enough to understand, the spelling and grammar aren't that important to me. The communication of one's ideas is what counts (to me). Most of these mistakes don't even apply to the language when spoken, so I try to treat the written form as equivalent.
Although occasionally a person's mistakes are so utterly atrocious that you can't even understand what they're saying, in which case intervention might be necessary.
What really
does bother me though (but which I still don't comment on) is blatantly incorrect use of periods. There is a person at another forum who uses maybe one comma every 10-15 posts. They use periods where commas should be used and it makes it
really hard to follow what they're saying. There are way too many abrupt stops which should be slight pauses and it makes the reading very choppy and annoying.